EP2367493B1 - Electrosurgical device having a temperature measurement device for determining a temperature and/or a temperature change at a neutral electrode - Google Patents

Description

The invention relates to an electrosurgical device according to claim 1.

In high-frequency surgery, electrical energy is supplied to the tissue to be treated. In this case, a distinction is generally made between a monopolar and a bipolar application of the high-frequency current (HF current).

In a monopolar application usually only one active electrode is provided, to which a high-frequency alternating voltage is applied. The active electrode is located, for example, on an electrosurgical instrument for cutting and / or coagulating tissue. Furthermore, it is necessary to attach a neutral electrode to the patient's body, which closes the circuit via the tissue lying between the active electrode and the neutral electrode. The shape of the active electrode depends on the particular application. The surface of the active electrode, through which alternating current is conducted into the tissue, is relatively small, so that in the immediate vicinity of the active electrode, a high current density and consequently also a high heat development arise.

The current density decreases strongly with the distance from the active electrode, unless high current densities occur at other parts of the body due to considerable differences in the fabric conductivity. The AC voltage applied to the active electrode is dissipated via the neutral electrode. It is important to ensure that the neutral electrode over a large area rests against the body of the patient and opposes the high-frequency alternating current only a low contact resistance.

In bipolar application, two active electrodes are provided between which the tissue to be treated is received. The current flow is closed by the tissue lying between the two active electrodes so that it is heated when an HF voltage is applied. A large part of the current flows between the two active electrodes.

It happens that the neutral electrode is not properly attached to the patient or partially peels off during treatment. In these cases, the current flow is limited to the remaining parts of the neutral electrode, which can lead to a significantly higher impedance at this and generally to a higher current density within the adjacent tissue. As the following reference to prior art documents shows, monitoring systems are known which allow an assessment of the mounting quality of the neutral electrode.

For example, is from the DE 10 2004 025 613 B4 A method for determining the junction impedance between two partial electrodes or electrode sections of a divided neutral electrode in high-frequency surgery is known. In this case, the transition impedances between the two electrode sections are determined by means of a resonant circuit. It is assumed that the transition impedance between the individual sections is considerably lower in the case of a large area contact of the neutral electrode.

Recently, treatment methods have been developed in which relatively high RF currents are applied over a longer period of time. The risk of tissue burning at the neutral electrode is increased in this process. Thus, even with a correctly applied neutral electrode, damage to the tissue may occur depending on the treatment method or course of treatment. Theoretically, it would be possible to further increase the contact surface of the neutral electrode, but this is often not practical.

Therefore, it is necessary to monitor the temperature at the neutral electrode. The US 2006/0079872 A1 provides a device for this purpose. According to this document, a resistor is coupled into the treatment stream, the heating of which can be monitored by means of a thermal sensor. The resistance should be chosen so that it essentially reflects real impedance relationships between the neutral electrode and the active electrode. However, a suitable choice of resistance is very difficult, since the impedance values in each application are dependent on the method used, the instrument, the positioning of the instrument and the neutral electrode, the organ to be treated, etc.

There are other approaches in which it is envisaged to provide commercial temperature sensors directly to the electrodes. The provision of these measuring devices to the electrodes is very complex. Furthermore, there are frequent localized warming events that may not be detected by these sensors.

Typically, the impedance between the two halves of a split neutral electrode is measured as previously described. This provides a guideline for the applied area since the resistance is anti-proportional to it. Furthermore, the current is measured via the neutral electrode and, together with the contact resistance, a theoretical power loss is estimated, which drops at the electrodes. This power loss can be compared with empirically determined limit values in order to draw conclusions about the temperature present at the neutral electrode. However, these approaches are very error-prone and can not provide safe protection against patient injury. Different tissue types are not considered here.

Based on this prior art, in particular of the EP 1 707 151 A2 , It is an object of the present invention to provide an electrosurgical device with an improved temperature measuring device. Furthermore, a corresponding method is to be provided for determining a temperature and / or temperature change at a neutral electrode. In particular, the method and the device should enable a safe and efficient assessment of the temperature conditions at the neutral electrode.

The object is achieved by an electrosurgical device according to claim 1.

A central idea is therefore to estimate the temperature of the neutral electrode or its temperature change on the basis of impedance measurements. For this purpose, the neutral electrode according to the invention has a contact agent layer which has a temperature-specific impedance. The electrical resistance of the contact agent layer thus changes depending on the present temperature. A temperature-specific impedance in the context of this invention means the change of the impedance as a function of the temperature within a relevant temperature range. For electrosurgery the relevant temperature range is between 10 and 100 ° C. Perhaps an interval of 20 to 70 ° C, in particular 20 to 60 ° C from.

Preferably, the contact agent layer will have such a material property that the impedance decreases with increasing temperature, in particular within the relevant interval. Local heating of the neutral electrode leads to a drop in the measured impedance. The impedance measuring device can thus always detect the lowest impedance and thus the section of the contact medium layer with the highest temperature.

The impedance measuring device comprises a measuring current generator, which is designed to provide a measuring current at a first electrode section and at a second electrode section. Thus, the neutral electrode is subdivided into at least a first electrode section and a second electrode section. The measurement of the impedance is ensured between these two electrode sections. It is conceivable to measure a plurality of impedances between a plurality of electrode sections. Thus, a better detail resolution of the temperature conditions is achieved at the neutral electrode. The neutral electrode thus serves not only for the application of the HF current, but also for the determination of the impedances or impedance ratios within or on the contact agent layer.

The measuring current generator can be designed to provide the measuring current with an alternating voltage, in particular with an alternating voltage with a frequency ≦ 300 kHz, in particular ≦ 150 kHz, in particular ≦ 100 kHz. With such low frequencies compared to those used in the treatment by means of the RF current, an effective measurement of the impedance can be made. It is conceivable to separate measuring currents and HF currents for treatment by corresponding filters from one another and to evaluate them separately.

The electrode sections are electrically isolated from one another on the contact middle layer. In order to generate different potentials at the individual electrode sections, it is necessary to form them in such a way that they are electrically insulated from one another.

The HF generator can be designed to provide an HF current with an AC voltage having a frequency ≥ 300 kHz, in particular ≥ 1000 kHz. Such frequencies are common in HF surgery and capable of performing advantageous coagulation and transection of tissue. These frequencies are significantly different from the frequencies used for the measurement currents. Frequency filters can be used to separate the measurement voltage from the RF voltage.

The contact means layer can have an electrical impedance with a high temperature dependence, in particular with a (relative) impedance change ≥ 1% per degree Celsius, in particular ≥ 2% per degree Celsius. The higher the temperature dependence of the contact agent layer used, the easier it is to detect a temperature change by means of the impedance change. Preferably, the relative impedance change in the relevant temperature range is greater than 1% per degree Celsius.

The contact agent layer may comprise or consist of hydrogel. Preferably, the contact agent layer is formed of hydrogel. When applying RF current, hydrogel is used to reduce the contact resistance between the electrodes and the skin. Hydrogel has a strong dependence on the temperature with respect to its impedance. Therefore, hydrogel is very well suited to carry out the temperature determination according to the invention. The hydrogel has a dual function in this case. For one, it serves the better Application of the HF current and / or mechanical attachment of the neutral electrode to the patient, on the other hand, it forms part of a temperature sensor.

The temperature measuring device may include an impedance integrating device configured to integrate impedance changes over a predetermined period of time to make heat balance estimates. By a long-term observation (temporal integration) of the impedance changes over all warm-up and cool-down phases in the course of an intervention, a real heat balance estimate can be made and thus a reliable statement about the thermal situation at the neutral electrode can be made.

The predetermined time period may include a plurality of activation and deactivation phases of the RF generator. Thus, both the heating during the activation phase - RF current is applied - as well as the cooling during the deactivation phase - no RF current is applied - are taken into account in the assessment of the temperature.

The electrosurgical device may comprise a recognition device for determining parameters, in particular at least one electrode surface of the neutral electrode and / or a temperature coefficient. The impedance values measured at a neutral electrode depend on a variety of factors. This includes the area of the neutral electrode, in particular the area of the electrode sections, their position relative to one another, the tissue resistance, etc. It is possible to store parameters relevant to the calculation of the temperature device-specifically, in particular neutral-electrode-specific. The recognition device can determine or read these parameters and process them in corresponding models or calculations.

The recognition device may include a database having a plurality of parameters and a plurality of neutral electrode types, wherein the recognition device is adapted to detect a connection of a particular type of neutral electrode and to read out the parameters from the database accordingly. The determination of the connected neutral electrode can therefore be automatic (eg via an RFID tag, which is located at the neutral electrode). Numerous other methods for determining the neutral electrode types are conceivable. It would also be possible, the neutral electrode types before treatment manually enter or make a determination of the relevant parameters in a given test position.

The electrosurgical device may comprise an interrupting device which is designed to interrupt or limit the HF current when a predetermined impedance change or when exceeding a predetermined temperature at the neutral electrode is exceeded. It is also conceivable that the interruption device emits a warning signal when falling below a predetermined impedance value.

The electrosurgical device can have a contact agent layer with such a material property that its impedance decreases with increasing temperature. That is, a material having a negative temperature coefficient can be used. Thus, it is possible to detect along the large-area neutral electrode sections with very low resistance due to particularly high temperatures.

The temperature measuring device for determining the temperature and / or the temperature change can take into account the effective value of the HF current, in particular of the applied HF current. For example, it is possible to calculate a relation between impedance change and effective value (eg ΔR / I _HF ) in order to determine a temperature change and / or a resistance, in particular a tissue resistance or a contact resistance between electrode and tissue. Among other things, the resistance can reveal how well a neutral electrode rests against the tissue.

The electrosurgical device comprises a current integrating device, which according to the invention is designed to sum up a value with respect to the HF current, in particular the RMS value, over time, in particular a predetermined period, and the sum for determining the temperature and / or the temperature change with an impedance change to relate to. It is simpler and less flawed if both the impedance change and the sum of the applied RF current are considered over a given time interval. The individual values can be related (eg, ΔR / ΣI _HF ) to capture system characteristics and determine temperature and / or temperature changes.

1. a) Bestimmen mindestens eines Impedanzwerts der Kontaktmittelschicht;
2. b) Berechnen einer Temperaturänderung und/oder einer Temperatur an der Neutralelektrode mindestens anhand des Impedanzwerts.

A method for determining a temperature and / or a temperature change at a neutral electrode with a contact agent layer is disclosed, wherein the method comprises the following steps:

1. a) determining at least one impedance value of the contact agent layer;
2. b) calculating a temperature change and / or a temperature at the neutral electrode based at least on the impedance value.

The method also makes use of the dependence of the impedance of the contact agent layer on the present temperature. Because of the immediate neighborhood between the contact agent layer and the applying part of the neutral electrode, as well as the neighborhood between the contact agent layer and the tissue, a quick heat exchange takes place. It can be considered that the tissue temperature of the tissue immediately below the neutral electrode is substantially equal to the temperature of the neutral electrode and the contact agent layer. This makes it possible to carry out a realistic estimation of the temperature balance at the neutral electrode. An inadmissibly high temperature increase due to the applied HF current can be detected and prevented.

Step a) may be performed multiple times during a plurality of activation and deactivation phases to determine a plurality of impedance values. Thus, the temperature profile or the individual temperature changes at the neutral electrode can be better assessed. A realistic estimate of the present temperature can be made.

In step b), the duration of the activation and / or deactivation phase and / or an effective value of the HF current can be taken into account.

Step b) may include integration over a plurality of impedance values over time.

The method may include detecting an impedance change during an activation and / or deactivation phase. For example, it is possible to use the quotient between cooling time and impedance change to draw conclusions about the present temperature. It can probably be assumed that the neutral electrode cools faster at a strong temperature gradient between the neutral electrode and the environment. The change in impedance relative to time can thus provide an important parameter for determining the temperature.

wobei

α :

ein spezifischer Temperaturkoeffizient,

T ₀:

eine Ausgangstemperatur,

R(T ₀):

eine Impedanz bei der Ausgangstemperatur T ₀,

R(T):

die gemessene Impedanz ist.

The calculation of the temperature change may comprise a linear estimation, in particular by means of the formula: $$\mathrm{\Delta }T=\frac{R\left(T\right)-R\left(T_0\right)}{\alpha *R\left(T_0\right)}.$$

in which

α :

a specific temperature coefficient,

T ₀ :

a starting temperature,

R ( T ₀ ):

an impedance at the starting temperature T ₀ ,

R ( T ):

the measured impedance is.

Although a linear relationship between temperature and impedance is not present in the contact means used, preferably hydrogel, the change in impedance as a function of temperature can be approximated sufficiently accurately by a linear equation. Alternatively, polynomial equations with a higher degree can be used for approximation. The specific temperature coefficient can be determined in advance in corresponding test positions. It is also conceivable to determine a multiplicity of temperature coefficients for a polynomial equation with a higher degree.

• Ein Detektieren eines bestimmten Neutralelektrodentypen der angeschlossenen Neutralelektrode;
• Eine Auswahl eines vorgegebenen Temperaturkoeffizienten oder eines beliebigen anderen Parameters gemäß dem Neutralelektrodentypen.

The method may include:

• Detecting a particular neutral electrode type of the connected neutral electrode;
• A selection of a given temperature coefficient or any other parameter according to the neutral electrode type.

Thus, previously determined parameters can be automatically included in the process.

The method may include outputting a warning signal and / or shutting off or shutting down the RF current if the measured impedance change exceeds a predetermined limit. Thus, in the event of an impermissible temperature, a warning message can be output, the HF current can be interrupted or limited in order to preclude any damage to the patient.

• Fig. 1 ein HF-Generatorsystem mit einem monopolaren Instrument;
• Fig. 2 Komponenten des HF-Generatorsystems;
• Fig. 3 Widerstands- und Stromverhältnisse an einer Neutralelektrode; und
• Fig. 4 ein Diagramm mit idealisierten Widerstands-Temperaturverläufen.

The invention will be described with reference to some embodiments, which are explained in more detail by means of illustrations. Hereby show:

• Fig. 1 an RF generator system with a monopolar instrument;
• Fig. 2 Components of the RF generator system;
• Fig. 3 Resistance and current conditions at a neutral electrode; and
• Fig. 4 a diagram with idealized resistance-temperature curves.

In the following description, the same reference numerals are used for the same and like parts.

The Fig. 1 shows an electrosurgical device comprising an RF generator system 30, a monopolar instrument 20 and a neutral electrode 10. The RF generator system 30 provides an RF current I _HF which is applied by means of the monopolar instrument 20 and the neutral electrode 10 to a torso 1 becomes. The Fig. 1 represents a schematic cross section through this torso 1. The neutral electrode 10 is attached to the torso 1 over a large area. The monopolar instrument 20 accommodates an active electrode which has a substantially smaller area than the neutral electrode 10. The current flows from the active electrode to the neutral electrode. In the immediate vicinity of the active electrode, the current density is so high that a targeted coagulation or separation of tissue 3 (see. Fig. 3 ) can be carried out.

Fig. 2 3 shows essential components of the HF generator system 30. These include a control device 36, a display device 32, an operating device 34 and a measuring device 37. The user of the electrosurgical device can activate or deactivate the HF current I _HF via the operating device 34. Furthermore, it is possible to set various modes of operation, such as one for cutting tissue and another for coagulating it. Depending on the inputs of the user, the controller 36 controls the RF generator 31, which provides an RF current I _HF according to the specifications. The display 32 may be used to display set parameters, such as the current operating mode. Furthermore, the display device 32 can display a temperature currently present at the neutral electrode 10 and issue warnings that protect the patient from undesired damage from the treatment. According to the invention, the temperature of the neutral electrode 10 is determined by the measuring device 37 using a secondary current source 38. Once the neutral electrode 10 has a temperature that could possibly lead to burns, the RF generator 31 is turned off and the display 32 issues corresponding warnings.

In one embodiment of the neutral electrode 10 according to the invention (see. Fig. 3 ) comprises a first electrode section 11 and a second electrode section 11 '. The electrode sections 11, 11 'are arranged on a carrier material in such a way that they are electrically insulated from one another.

In one embodiment of the neutral electrode 10 is located between the individual electrode sections 11, 11 ', an electrical insulator or hydrogel 13. In the present embodiment, it is a self-adhesive neutral electrode 10 having a layer of electrically conductive hydrogel 13 and for application of the HF Current I _HF is glued to a fabric 3. The present invention takes advantage of the fact that the hydrogel 13 has a strong temperature coefficient of impedance. For example, commercially available neutral electrodes 10 were measured with commercial hydrogel 13 in the temperature range of 25 ° C to 40 ° C, a relative impedance change of 2 to 4% per degree Celsius. This effect can be used to determine the temperature increase at the neutral electrode 10. Here, however, various other parameters must be considered. For example, the environmental conditions have a strong influence on the measured impedance R (T).

In order to measure the impedance R (T) dependent on the temperature T, the measuring device 37 comprises the secondary current source 38. This device provides a measuring current I _measuring which is applied to the electrode sections 11, 11 '. By means of a parallel to the secondary power source 38 connected voltage measuring device 39 can be a _measurement voltage U detected _measurement . Thus, the measuring device 37 can measure an overall impedance. In a first model it is assumed that this total impedance, as in the Fig. 3 shown, composed of several resistors. Thus, the measuring current I _measurement of the first electrode section 11 passes through the hydrogel 13, at least partially enters the tissue 3, again passes through the hydrogel 13 and reaches the second electrode section 11 '. The total impedance is thus composed at least of a gel resistance R _Gel1 , a tissue _resistance R _tissue and a second gel resistance R _Gel2 .

In the first model it can be assumed that the tissue resistance change in the relevant temperature range (about 20 to 70 °) can be neglected. The measuring device 37 can determine the gel resistances R _Gel1 , R _Gel2 by means of the measuring current I _Mess . The tissue resistance R _tissue can be determined by further measurements or set to a constant value which corresponds to approximate resistances commonly encountered in tissue.

In a second model, it is assumed that the gel resistances R _Gel1 , R _{Gel2 are} less than the tissue _resistance R _tissue , so that the measurement of the measuring device 37 exclusively detects the changes in the impedance R (T) of the hydrogel 13. It is possible to select a hydrogel 13 accordingly.

In a third model, which probably best models the reality when using a conventional hydrogel 13, it is assumed that the resistance of the hydrogel 13 is greater than that of the tissue 3. In particular, because of the small layer thickness of the hydrogel 13, this can be done in reality often occur. Experiments have shown that 30% of the flow of current takes place within the hydrogel layer while 70% of the flow of current takes place in the tissue. Situations are conceivable in which only about 10% of the flow of current takes place in the hydrogel 13. The impedance R (T) thus settles, as in Fig. 3 shown, from the gel resistors R _Gel1 , R _Gel2 and the tissue _resistance R _tissue together. Since the tissue temperature in the application of the HF current I _HF compared to the temperature of the hydrogel 13 only changed very slowly - the blood circulation leads to a fast removal of the accumulating heat energy - can be assumed in this model, a constant or approximately constant value for R _tissue . On the impedance change Δ _R , the temperature of the fabric 3 has only a small influence. This can thus be detected according to the invention.

Since the gel resistances R _Gel1 , R _{Gel2 decrease sharply} with increasing temperature T, a further advantageous effect arises. If the neutral electrode 10 is locally or locally heated, a sharp drop in the measured impedance R (T) in this region can be detected.

The thermal effect that occurs in both the fabric 3 and the hydrogel 13 and at the neutral electrode 10 is due to the applied RF current I _HF . When two electrode sections 11, 11 'are used, this HF current I _HF divides into two HF _substreams I _HF1 , I _HF2 . These HF partial currents I _HF1 , I _HF2 are shown schematically in _FIG Fig. 3 shown.

In this embodiment, it is considered that the relationship between the impedance R (T) of the hydrogel 13 and its temperature T can be modeled sufficiently accurately with a first-order temperature coefficient α . Alternatively, higher order temperature coefficients can be added.

Mathematically, the temperature change Δ T is as follows: $$\mathrm{\Delta }T=\frac{R\left(T\right)-R\left(T_0\right)}{\alpha *R\left(T_0\right)}$$

Here, R (T) is the measured impedance at the temperature T, R (T ₀ ) is an impedance at an output temperature T ₀ and α is the specific temperature coefficient. The specific temperature coefficient α can be determined, for example, within a test setup.

Look at the diagram Fig. 4 the operation of the measuring device 37 according to the invention will be described.

The X-axis indicates the course of time t in seconds. The Y-axis gives impedance values R ₁ , R ₂ , R ₃ , R _{4 of} a measured impedance R (T) in ohms (lower Line) and a temperature T (t) (upper line) present at the neutral electrode 10 in degrees Celsius. While the temperature values T ₁ , T ₂ , T ₃ , T ₄ decrease in the Y direction, the impedance values R ₁ , R ₂ , R ₃ , R _{4 increase} along the Y direction.

The diagram shows an example of the course of an HF treatment with a neutral electrode 10 according to the invention. Immediately after the application of the neutral electrode 10, the first temperature value T ₁ is established in the hydrogel 13. The first temperature value T ₁ essentially corresponds to the body surface temperature of approximately 32 ° C. The measuring device 37 can detect the first impedance value R ₁ . At time t ₁ , the HF generator 31 is activated with a low power (shown schematically by the ramp in the diagram). The activation phase lasts until time t ₂ . During the activation phase, the measured impedance R (T) drops to the impedance value R ₃ . Since the measuring device 37, the output temperature T ₁ , the output impedance R ₁ and the impedance value R ₃ at time t _{2 is} known, it can calculate the temperature change .DELTA.T by means of the above-mentioned formula. Based on this, the absolute temperature value T ₃ can be determined.

During a deactivation phase (time t ₂ to t ₃ ), the measured impedance R (T) increases. Again, the temperature change Δ T can be determined based on the impedance change ΔR, since R _{2 is} measurable and R ₃ , T _{3 are} known. The measuring device 37 can therefore calculate the temperature T ₂ from the current temperature change ΔT. In a subsequent activation phase of the HF generator 31 (time period t ₃ to t ₄ ), the temperature of the neutral electrode T (t) rises again. Again, the temperature change ΔT can be calculated.

An exemplary embodiment for determining the temperature T (t) according to the invention and the temperature change ΔT at the neutral electrode 10 has been described above.

In further embodiments, further parameters can be processed. For example, it is conceivable to take into account the temperature change ΔT during a time interval. Thus, a strong drop in temperature during a relatively short deactivation phase can be used as an indicator that a relatively high temperature T (t) is present at the neutral electrode 10, since there is probably a strong temperature gradient to the environment. Numerous other methods are conceivable which exploit the effect that a direct correlation between the Impedance change Δ R of the hydrogel 13 and its temperature change Δ T is present.

LIST OF REFERENCE NUMBERS

1

torso

3

tissue

10

neutral electrode

11, 11 '

electrode section

13

hydrogel

20

monopolar instrument

30

RF generator system

31

RF generator

32

display

34

operating device

36

control device

37

measuring device

38

Secondary power source

39

Voltage measuring device

.DELTA.R

impedance change

.DELTA.T

temperature change

T (t)

Temperature of the neutral electrode

R (T)

impedance

I _mess

measuring current

U _Mess

measuring voltage

R _Gel1 , R _Gel2

Gelwiderstand

R _tissue

tissue resistance

I _HF

RF power

I _HF1 , I _HF2

RF partial flow

t ₁ , t ₂ , t ₃ , t ₄ , t ₅

time

T ₁ , T ₂ , T ₃ , T ₄

temperature value

R ₁ , R ₂ , R ₃ , R ₄

impedance values

T

temperature

α

temperature coefficient

Claims (14)

1. Electrosurgical device, comprising:

   - an RF generator (31) for generating an RF current (I_HF), which can be introduced into biological tissue (3) via an instrument (20) and a neutral electrode (10) with a contact agent layer (13) ;

   - a temperature measuring apparatus (37) for determining a temperature and/or a change in temperature (ΔT) at the neutral electrode (10); the temperature measuring apparatus (37) comprising an impedance measuring apparatus for determining the temperature and/or the change in temperature (ΔT), said impedance measuring apparatus being embodied to register an impedance (R(T)) of the contact agent layer (13),

   the impedance measuring apparatus comprising a measurement current generator, which is embodied to provide a measurement current (I_Mess) at a first electrode section (11) and a second electrode section (11'),
   characterized by
   a current integration apparatus, which is embodied to sum a value in respect of the RF current (I_HF) over time and to relate this value to a change in impedance (ΔR) in order to determine the temperature and/or the change in temperature (ΔT).
2. Electrosurgical device according to Claim 1,
   characterized in that
   the measurement current generator is embodied to provide the measurement current (I_Mess) with an AC voltage, in particular with a frequency of less than or equal to 300 kHz, in particular of less than or equal to 150 kHz, in particular of less than or equal to 100 kHz.
3. Electrosurgical device according to one of the preceding claims,
   characterized in that
   the electrode sections (11, 11') are arranged on the contact agent layer (13) in a manner electrically insulated from one another.
4. Electrosurgical device according to one of the preceding claims,
   characterized in that
   the RF generator (31) is embodied to provide an RF current (I_HF) with an AC voltage with a frequency of greater than or equal to 300 kHz, in particular of greater than or equal to 1000 kHz.
5. Electrosurgical device according to one of the preceding claims,
   characterized in that
   the contact agent layer (13) has an electrical impedance with a temperature dependence, in particular with a relative change in impedance of greater than or equal to 1% per degree centigrade, in particular of greater than or equal to 2% per degree centigrade.
6. Electrosurgical device according to one of the preceding claims,
   characterized in that
   the contact agent layer (13) comprises a hydrogel.
7. Electrosurgical device according to one of the preceding claims,
   characterized in that
   the temperature measuring apparatus (37) comprises an impedance integration apparatus, which is embodied to integrate changes in impedance (ΔR) over a predetermined period of time in order to perform a heat balance estimate.
8. Electrosurgical device according to Claim 7,
   characterized in that
   the predetermined period of time comprises a multiplicity of activation and deactivation phases of the RF generator (31).
9. Electrosurgical device according to one of the preceding claims,
   characterized by
   identification equipment for determining parameters, in particular of at least one electrode area of the neutral electrode (10) and/or of a temperature coefficient (α).
10. Electrosurgical device according to Claim 9,
    characterized in that
    the identification equipment comprises a database with a multiplicity of parameters and a multiplicity of neutral electrode types, the identification equipment being embodied to register a specific neutral electrode type being connected and accordingly read out the parameters from the database.
11. Electrosurgical device according to one of the preceding claims,
    characterized by
    a cut-off apparatus, which is embodied to cut off or limit the RF current (I_HF) when a predetermined change in impedance is exceeded.
12. Electrosurgical device according to one of the preceding claims,
    characterized in that
    the contact agent layer (13) has such a material property that the impedance thereof decreases with increasing temperature.
13. Electrosurgical device according to one of the preceding claims,
    characterized in that
    the temperature measuring apparatus (37) for determining the temperature and/or the change in temperature takes the effective value of the RF current (I_HF) into account.
14. Electrosurgical device according to one of the preceding claims,
    characterized in that
    the current integration apparatus is embodied to sum the effective value of the RF current (I_HF) over time.

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